lib min_heap: Switch to size_t

size_t is the correct type for a count of objects that can fit in
memory: this also means heaps now have the same memory layout as darrays
(fs/bcachefs/darray.h), and darrays can be used as heaps.

Cc: Kuan-Wei Chiu <visitorckw@gmail.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Coly Li <colyli@suse.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>